Rats replay their visual experiences in their sleep

The storage of memories in the brain is thought to involve the process of long-term potentiation (LTP). LTP happens when nerves fire repeatedly in response to a stimulus; this causes them to permanently alter their behavior and connections to other nerves. But this raises an obvious question: how do we ever remember a situation that happens only once? Single events aren't going to be sufficient on their own to trigger LTP. It was proposed that memories are generated in part by having the brain mentally repeat the event separate from its occurrence, a process that may include dreams. There's been some suggestive experimental support for this, but a recent paper goes much further in examining this replay process.

Using lab rats, the researchers implanted devices that allowed them to register nerve activity in two areas of the brain: the visual cortex, which integrates visual input, and the hippocampus, where memories are consolidated. Sending the rats around a figure-eight maze allowed them to note that the firing of some neurons in the visual cortex was consistently associated with the rat's position within the maze. This sort of spatial/visual map has been seen before in other studies. But the authors were able to show that a similar spatial map exists in the hippocampus—neurons there also consistently fired when the rats reached specific areas of the maze.

The big result, however, came when they let the rats sleep. In the visual cortex, the electrodes were able to detect rapid replays of the same sequences of nerve firings that occurred as the rats went through the maze. Although these cases were rare, they occurred far more frequently than would be expected from chance. But it wasn't just the visual system that was replaying the past: these replays were also associated with a similar replay in the hippocampus at rate that is well above chance. Thus, the idea that memories are created by repeated nerve firings, and that many of these repeats occur during sleep, has gotten some significant experimental support.

The authors suggest that this process is rarely an exact repeat of history. As they relaxed the standards for a spatial match, the frequency of similar nerve firings increased dramatically. They suggested that being able to read from more neurons would allow them to identify many more of these partial matches. Even without that improvement, though, the data look pretty impressive.